Surface temperatures

The temperatures of surfaces in a room vary more than air temperature and have considerable effect. A window may be as low as 5°C and a radiator as high as 75-80°C.
Of course, these different temperatures produce an overall average but the effect on the body depends on their relative positions. If there is a radiator at 75°C close to you on one side and a window of the same area at 5°C equidistant on the other, the average temperature of the two surfaces would be 40°C. This average is known as the mean radiant temperature (MRT) and in practice would also be influenced by all the other surface temperatures in the room, not just the window and radiator.
If you move closer to the window the radiator has less effect and the MRT at your new position may go down to, say, 20°C. Radiation loss from the body depends on the MRT at the position occupied, while convection loss depends on the air temperature.
Comfort temperatures
Although people vary in their comfort requirement, most feel comfortable if the average of air temperature and MRT is in the range 19°C to 23°C. If, for example, all surfaces were at 18°C for simplicity, as in Fig. 1.3, the MRT anywhere in the room will be 18°C. With an air temperature of 24°C, the average is 21°C. If you are comfortable at this combination, you would also be comfortable with the air at 20°C and MRT at 22°C because the average would also be 21 °C.
This average, between MRT and air temperature, is used on the continent as a measure of comfort. It was devised by the Frenchman Missenard in the 1930s and is often shown in °M. The institutions concerned with heating in this country have recently adopted this as the scale for comfort but have called it the ‘dry resultant temperature’; I find it easier and more descriptive to call it the ‘comfort temperature’.
Inside comfort conditions depend on a combination of radiant and air temperatures, while those outside depend on the sun’s radiation and air temperature. This outside combination is called the sol-air temperature. It is used in air-conditioning calculations but the solar effect is so small and unpredictable in winter that it is ignored in heating calculations and only the outside air temperature is used.
These comfort concepts explain many little-understood and often-argued points. It is not necessary to know about these in order to plan a simple heating system and install it; you could pass straight on to for this, but I want to explain more than is usual so that you will
know your oil heating needs repair and the solutions better than most, and be better able to judge claims you might hear for this or that method, system or component.
If we want to be comfortable, we must achieve the comfort temperature we want. But we can- not measure comfort temperature; there are no thermometers readily available to record it. How can we find out what it is?
To make things simpler I have produced Table 1.2, which enables you to find approximate comfort temperature as a percentage of air temperature. Because the ceiling and floor effects tend to cancel out in any room in a normal house and to simplify things on the two-dimensional diagrams, I will ignore the small effect on MRT of these surfaces; the average U-value referred to is therefore that of all the walk, including windows.
In Fig. 1.4 various room conditions are illustrated; the comfort temperature is shown at nine different positions. In the first two I have also shown the MRT above each comfort temperature. The comfort temperature is the average of the MRT at each position, and the air temperature.
These comfort temperatures were worked out by computer using many factors but if you check the centre point in each room, you will find that the simplified table I have designed gives a close indication of the general comfort level represented by the comfort temperature at the centre of the room.
Where the comfort temperature is below 19°C the average person will feel too cold. To raise the comfort temperature it would be necessary to raise the air temperature, which uses more fuel.
You can see now why people are often fiddling with the room thermostat especially with a warm-air system. It is no good saying ‘the system has been properly designed; leave the thermo- stat alone’. It may have been properly designed but on a day when the outside surfaces are cold, you need a higher air temperature to compensate and give comfort.
If the next day is very sunny, the outside walls warm up, the surface temperatures rise, and you turn down the thermostat because now you feel too warm. As you turn the thermostat down you glance at the thermometer and see that it shows exactly the same air temperature as the day before. Now you suspect the thermometer is faulty as well as the heating.
To improve matters we need to have surfaces which do not fluctuate so much with changes of outside temperature and to be able to use the air thermostat to control our comfort level with a minimum of correction.
To expect a thermostat to keep you comfortable without adjustment is asking too much. It is designed only to keep the air at a particular temperature; it cannot sense radiant heat as we can. Also a warm-air system heats only the air; it has little effect on the surface temperatures which affect us. Even with radiators much of the heat emitted is by convection.
Until someone designs a human ‘comfort stat’ the best thing is to create an environment where an acceptable comfort temperature can be achieved without a much higher air temperature so that the thermostat reading is more closely related to the comfort temperature.
You can now see that the type of heating you have, the position of radiators, double glazing and insulation all have an effect on surface temperatures and therefore on comfort levels.

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